any idea with the following math questions?

​

https://preview.redd.it/8u57n8epiaib1.png?width=781&format=png&auto=webp&s=8f1249918391939167f7c9a37ba71d9e24834d1d

why is the answer A?

https://preview.redd.it/71cby9k5waib1.png?width=794&format=png&auto=webp&s=7e478997e9f107a78430f10873957bb0b78f0cbc

if i follow the previous method i get answer B but the correct answer is A, why?

explaination given :

https://preview.redd.it/ht2dl3cmwaib1.png?width=833&format=png&auto=webp&s=33c42e345819f8e00ac5ef207b7f75c0c7168727

​

https://preview.redd.it/vlgsax4mtaib1.png?width=786&format=png&auto=webp&s=aaedc28302fdaf8c7c57c588654aa1bef69ddb2c

why is the answer C?